This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-263591, filed Aug. 31, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a waveform measuring method and apparatus. More particularly, the present invention relates to a waveform measuring method and apparatus for obtaining a signal waveform of a signal under test having an arbitrary repetition cycle outputted from a signal under test generator as a test object.
2. Description of the Related Art
In general, a signal under test generator for generating an electrical signal, an optical signal or the like each having an arbitrary repetition cycle incorporates a reference signal oscillator for generating a reference signal that has a reference frequency xe2x80x9cfsxe2x80x9d and a waveform pattern generating portion for generating a waveform pattern of a signal waveform.
In such a signal under test generator, a reference signal outputted from the reference signal generator is employed so as to generate a repetition frequency signal that has a specified repetition frequency xe2x80x9cfaxe2x80x9d. In addition, this repetition frequency signal and a waveform pattern outputted from the waveform pattern generating portion are employed so as to generate an electrical signal or an optical signal each having an arbitrary repetition cycle xe2x80x9cTaxe2x80x9d.
The electrical signal and optical signal each having an arbitrary repetition cycle Ta outputted from such a signal under test generator are generally incorporated in an information communication system, and, for example, are employed as test signals of a variety of communication devices including an optical transmission cable.
Therefore, it is required to measure characteristics of the electrical signal or optical signal outputted from the signal under test generator in detail prior to testing a variety of communication devices containing the optical transmission cable incorporated in the information communication system.
One of the characteristic measurements of the electrical signal or optical signal is a signal waveform measurement.
Conventionally, there is proposed a variety of measuring techniques for measuring a signal waveform of a signal under test such as the electrical signal, optical signal or the like each having this arbitrary repetition cycle Ta.
However, in the case where the repetition cycle Ta of the signal under test, i.e., the repetition frequency xe2x80x9cfaxe2x80x9d is a high frequency signal that exceeds 10 GHz, the signal waveform of such a signal under test cannot be directly observed on a display screen such as oscilloscope. Thus, the selection range of the waveform measuring technique itself is limited.
A typical technique of measuring a signal waveform of a signal under test of which the repetition frequency xe2x80x9cfaxe2x80x9d exceeds 10 GHz will be described with reference to FIGS. 7A, 7B, and 7C.
As shown in FIGS. 7A and 7B, a signal under test xe2x80x9caxe2x80x9d having a repetition cycle Ta (for example, repetition frequency xe2x80x9cfaxe2x80x9d=10 GHz) is sampled by a sampling signal xe2x80x9cbxe2x80x9d having a cycle Tb (for example, repetition frequency xe2x80x9cfbxe2x80x9d=999.9 MHz) longer than the repetition cycle Ta of this signal under test xe2x80x9caxe2x80x9d.
In this case, a mutual relationship between the repetition periods Ta and Tb is adjusted, whereby the sampling position of a sampling signal xe2x80x9cbxe2x80x9d in the signal waveform in the repetition cycle Ta of the signal under test xe2x80x9caxe2x80x9d is shifted by a differential time xcex94T together with an elapse of time, as shown in FIGS. 7A and 7B, and is provided so as to be delayed as xcex94T, 2xcex94T, 3xcex94T, 4xcex94T, 5xcex94T, 6xcex94T . . . .
Therefore, a signal under test xe2x80x9ccxe2x80x9d after sampled by this sampling signal xe2x80x9cbxe2x80x9d is obtained as discrete waveform in which a pulse shaped waveform is generated at a position synchronized with the sampling signal xe2x80x9cbxe2x80x9d, as shown in FIG. 7C.
Then, the enveloped waveform of each pulse shaped waveform is obtained as a signal waveform xe2x80x9cdxe2x80x9d extended in a time axis direction of the signal under test xe2x80x9caxe2x80x9d.
A waveform measuring apparatus for measuring a signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d is configured as shown in FIG. 8, for example, based on the principle of the sampling technique shown in FIGS. 7A, 7B, and 7C.
The signal under test xe2x80x9caxe2x80x9d having the repetition frequency xe2x80x9cfaxe2x80x9d (repetition cycle Ta) is inputted to a sampling circuit 1 and a frequency divider 2.
The frequency divider 2 delivers to a phase comparator 3 an output signal obtained by frequency dividing the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d into 1/n.
A voltage control oscillator (VCO) 4 functions as a phase locked loop (PLL) for generating a signal that has a frequency (fa/n) of 1/n (n: positive integer) of the repetition frequency xe2x80x9cfaxe2x80x9d so as to feed the signal back to the phase comparator 3.
The phase comparator 3 configuring the phase locked loop (PLL) detects a phase difference between a phase of an output signal of the voltage control oscillator (VCO) 4 and an output of the frequency divider 2, and delivers a phase difference signal to the voltage control oscillator (VCO) 4.
A phase of an output signal from the voltage control oscillator (VCO) 4 is synchronized with a phase of the signal under test xe2x80x9caxe2x80x9d by means of this phase locked loop (PLL).
A frequency (fa/n) of an output signal having a frequency (fa/n) outputted from the voltage control oscillator (VCO) 4 is converted into a frequency of (fa/n)xe2x88x92xcex94f by means of a next stationary frequency divider 5a and a stationary multiplier 5b, and the converted frequency is inputted to a sampling signal generator circuit 6.
Here, the sampling signal generator circuit 6 applies to the sampling circuit 1 a repetition frequency (fb) as shown in formula (1) synchronized with an inputted output signal and a sampling signal xe2x80x9cbxe2x80x9d having a repetition cycle (Tb).
fb=(fa/n)xe2x88x92xcex94fxe2x80x83xe2x80x83(1)
Tb=(nTa)+xcex94Txe2x80x83xe2x80x83(2)
However, a relationship between xcex94f and xcex94T is shown in formula (3)
xcex94f/xcex94T=fa2/n2xe2x80x83xe2x80x83(3)
Then, the sampling circuit 1 samples the inputted signal under test xe2x80x9caxe2x80x9d by means of the sampling signal xe2x80x9cbxe2x80x9d inputted from the sampling signal generator circuit 6, thereby delivering the sampled signal under test xe2x80x9ccxe2x80x9d to a next signal processing/waveform display portion 7.
This signal processing/waveform display portion 7 calculates an enveloped waveform of the inputted signal under test xe2x80x9ccxe2x80x9d after sampled, and a scale of a time axis of this enveloped waveform is converted into a scale of the original signal under test xe2x80x9caxe2x80x9d, whereby the signal waveform xe2x80x9cdxe2x80x9d of the original signal under test xe2x80x9caxe2x80x9d is outputted to be displayed.
In this case, a magnification ratio of the signal under test xe2x80x9caxe2x80x9d of the measured, enveloped waveform to the signal waveform xe2x80x9cdxe2x80x9d is obtained as (fa/nxcex94f).
In the case where the signal under test xe2x80x9caxe2x80x9d is an optical signal instead of an electrical signal, this optical signal is applied to the frequency divider 2 after converted into an electrical signal.
In addition, in the case where the signal under test xe2x80x9caxe2x80x9d is an optical signal instead of an electrical signal, an electro-absorption modulator, for example, is employed instead of the sampling circuit 1.
This electro-absorption modulator is applied a pulse shaped electric field that is a sampling signal, thereby making it possible to sample the pulse shaped signal under test xe2x80x9caxe2x80x9d that is an optical signal that is inputted to the electro-adsorption modulator.
Then, the signal under test xe2x80x9ccxe2x80x9d that is the thus sampled optical signal is delivered to a signal processing/waveform display portion 7 after converted into an electrical signal.
However, a conventional waveform measuring apparatus employing a sampling technique shown in FIG. 8 has the following problems to be solved.
That is, an output signal from a multiplier 5b for generating a sampling signal xe2x80x9cbxe2x80x9d that has a repetition frequency fb=(fa/n)xe2x88x92xcex94f outputted from the sampling signal generator circuit 6 is produced by means of a phase locked loop (PLL) circuit composed of a frequency divider 2 for frequency dividing the signal under test xe2x80x9caxe2x80x9d, a phase comparator 3, and a voltage control oscillator (VCO) 4.
That is, the sampling signal xe2x80x9cbxe2x80x9d is produced by processing the signal under test xe2x80x9caxe2x80x9d targeted to be measured, and thus, such sampling signal xe2x80x9cbxe2x80x9d is always phase synchronized with the signal under test xe2x80x9caxe2x80x9d.
Therefore, a generated quantity of jitter in the signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d displayed at the signal processing/waveform display portion 7 is restricted, and thus, the measurement precision of the signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d is improved.
However, the repetition frequency xe2x80x9cfbxe2x80x9d of the sampling signal xe2x80x9cbxe2x80x9d is expressed by a function of the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d, as is evident from formulas (1) and (3) described previously.
This fact means that the repetition frequency xe2x80x9cfbxe2x80x9d of the sampling signal xe2x80x9cbxe2x80x9d cannot be arbitrarily set independently of the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d when a frequency dividing rate and a frequency multiplied rate are fixed.
That is, when the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d changes, the time resolution, i.e., measurement precision of the signal waveform xe2x80x9cdxe2x80x9d of the measured signal under test xe2x80x9caxe2x80x9d automatically changes.
Therefore, in this waveform measuring apparatus, the signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d cannot be measured with an arbitrary time resolution.
In addition, there is a problem that the selection range of the repetition frequency xe2x80x9cfbxe2x80x9d of the sampling signal xe2x80x9cbxe2x80x9d is greatly limited due to the specification characteristics or the like of each of the frequency dividers 2 and 5a. 
The present invention has been made in order to solve the foregoing problem. It is an object of the present invention to provide a waveform measuring method and apparatus capable of arbitrarily setting a frequency of a sampling signal for sampling a signal under test outputted from a signal under test generator targeted for testing independently of a repetition frequency of a signal under test while a sampling signal is always phase synchronized with a signal under test by generating a repetition cycle of a signal under test and a cycle of a sampling signal by means of a common reference signal, and capable of improving measurement precision of a signal waveform of a signal under test while a generated quantity of jitter is restricted, and capable of measuring the signal waveform with arbitrary resolution precision.
In order to achieve the foregoing object, according to a first aspect of the present invention, there is provided a waveform measuring method for generating a sampling signal that has a cycle longer than a repetition cycle of a signal under test outputted from a signal under test generator from a sampling signal generator circuit, sampling the signal under test by means of the sampling signal, and obtaining an enveloped waveform of the signal under test, the waveform measuring method comprising the steps of:
applying a common reference signal from the same reference signal generating portion to a reference signal input terminal of the signal under test generator and a reference signal input terminal of a sampling signal generator circuit;
setting to the sampling signal generator circuit a sampling frequency such that a desired delay time can be obtained relevant to a phase of the signal under test; and
generating the sampling signal having a cycle that corresponds to the sampling frequency based on the common reference signal and the sampling frequency in the sampling signal generator circuit,
wherein a repetition cycle of the signal under test and a repetition cycle of the sampling signal are set based on a cycle of the common reference signal so that the repetition cycle of the sampling signal can be set independently of the repetition cycle of the signal under test.
In this way, in the waveform measuring method, in the case where a reference signal oscillator for generating a reference signal that has a reference frequency employed for generating a repetition frequency of a signal under test is not provided in a signal generator targeted for testing, the signal generator outputting a signal under test of which a signal waveform is measured, an externally applied reference signal is employed to the reference signal input terminal.
Then, a common reference signal outputted from a reference signal oscillator provided at the outside of a signal under test generator is supplied to sampling signal generating means and a signal generator.
Therefore, in this case, the sampling signal and signal under test always maintain a phase synchronized state.
Moreover, the sampling signal is obtained by processing a reference signal instead of processing a signal under test.
Hence, with a simple configuration, a repetition frequency of a sampling signal for sampling a signal under test can be arbitrarily set independently of a repetition frequency of a signal under test, and a phase synchronization with a signal under test is always obtained. Thus, jitter generation is restricted.
In addition, in such a waveform measuring method, in the case where a reference signal oscillator for generating a reference signal that has a reference frequency employed for generating a repetition frequency of a signal under test is provided in a signal generator targeted for testing, the signal generator outputting a signal under test of which a signal waveform is measured, a sampling signal is generated by employing a reference signal outputted from a reference signal oscillator provided in a signal generator targeted for testing, contrary to the above description.
Therefore, in this case, the signal under test and sampling signal are generated by employing a common reference signal outputted from the same reference signal oscillator, thus making it possible to achieve the same advantageous effect as the above described case.
In order to achieve the foregoing object, according to a second aspect of the present invention, there is provided a waveform measuring apparatus having a sampling signal generator circuit for generating a sampling signal that has a cycle longer than a repetition cycle of a signal under test outputted from a signal under test generator; a sampling portion for sampling the signal under test by means of a sampling signal from the sampling signal generator circuit; and a data processing portion for obtaining an enveloped waveform of the signal under test sampled by means of the sampling portion, thereby obtaining a signal waveform of the signal under test, the measuring apparatus comprising:
a reference signal generating portion for applying a common reference signal to a reference signal input terminal of the signal under test generator and a reference signal input terminal of the sampling signal generator circuit; and
a sampling frequency setting portion for setting to the sampling signal generator circuit a sampling frequency such that a desired delay time can be obtained relevant to a phase of the signal under test,
wherein, in the sampling signal generator circuit, when the sampling signal having a cycle that corresponds to the sampling frequency is generated based on the common reference signal and the sampling frequency, the repetition cycle of the signal under test and the repetition cycle of the sampling signal are set based on a cycle of the common reference signal so that the repetition cycle of the sampling signal can be set independently of the repetition cycle of the signal under test.
In the thus configured waveform measuring apparatus, a reference signal oscillator for generating a reference signal that has a reference frequency employed for generating a repetition frequency of a signal under test is not provided in a signal generator targeted for testing, the signal generator outputting a signal under test of which a signal waveform is measured, and a reference signal externally applied to a reference signal input terminal of a signal generator targeted for testing is employed.
Then, a common signal outputted from the same reference signal oscillator provided at the waveform measuring apparatus is applied to sampling signal generating means and a signal under test generator.
Therefore, a sampling signal and a signal under test always maintains a phase synchronized state.
Moreover, a sampling signal is obtained by processing a reference signal instead of processing a signal under test.
Hence, with a simple configuration, a repetition frequency of a sampling signal for sampling a signal under test can be arbitrarily set independent of a repetition frequency of a signal under test. In addition, a phase synchronization is always obtained relative to a signal under test, and thus, jitter generation is restricted.
In addition, in order to achieve the foregoing object, according to a third aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the second aspect, further comprising a frequency measuring portion for measuring a repetition frequency of the signal under test, based on the reference signal output from the reference signal oscillator, wherein the measurement value of the repetition frequency of the signal under test measured by means of the frequency measuring portion is assigned to the sampling frequency setting portion.
In addition, in order to achieve the foregoing object, according to a fourth aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the second aspect, the waveform measuring apparatus further comprising:
a power divider for, when the signal under test is an optical signal, power dividing the signal under test that is the optical signal into two ways;
a photo detector for converting into a signal under test of an electrical signal a signal under test that is one of the optical signals divided by the power divider; and
a frequency measuring portion for measuring a repetition frequency of a signal under test converted into an electrical signal by means of the photo detector, based on the reference signal output from the reference signal oscillator,
wherein the measurement value of the repetition frequency of the signal under test measured by means of the frequency measuring portion is assigned to the sampling frequency setting portion.
In addition, in order to achieve the foregoing object, according to a fifth aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the second aspect, further comprising:
a power divider for, when the signal under test is an optical signal, power dividing the signal under test that is the optical signal in two ways;
a clock recovery for detecting a clock of the repetition cycle from one signal under test divided by the power divider, thereby converting the signal under test that is the optical signal into a signal under test of an electrical signal having the repetition frequency and outputting the converted electrical signal; and
a frequency measuring portion for measuring a repetition frequency of a signal under test converted into an electrical signal by the clock recovery, based on the reference signal output from the reference signal oscillator,
wherein the measurement value of the repetition frequency of the signal under test measured by means of the frequency measuring portion is assigned to the sampling frequency setting portion.
In addition, in order to achieve the foregoing object, according to a sixth aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the second aspect, the waveform measuring apparatus further comprising:
a photo detector for light receiving a signal under test of an optical signal sampled by means of a sampling signal of which a signal under test being the optical signal is inputted from the sampling signal generator circuit, and converting into a signal under test of an electrical signal as a signal under test that is an optical signal after sampled;
an analog/digital converter for converting a signal under test converted into an electrical signal by means of the photo detector into a digital signal under test, thereby delivering the converted digital signal under test to the data processing portion; and
a display for converting a scale of a time axis of the enveloped waveform obtained by means of the data processing portion into a scale of an original signal under test, and displaying the converted scale as a signal waveform of the signal under test.
In addition, in order to achieve the foregoing object, according to a seventh aspect of the present invention, there is provided a waveform measuring apparatus having a sampling portion having a reference signal oscillator, for sampling a signal under test by means of a sampling signal having a cycle longer than a repetition cycle of the signal under test outputted from a signal under test generator for generating a signal under test having a repetition cycle set by employing a reference signal generated by the reference signal oscillator; and a data processing portion for obtaining an enveloped waveform of a signal under test sampled by this sampling portion, thereby obtaining a signal waveform of the signal under test from this enveloped waveform, the waveform measuring apparatus comprising:
a sampling signal generator circuit having a reference signal input terminal wherein a reference signal common to a reference signal generated by the reference signal oscillator of the signal under test generator is applied to the reference signal input terminal; and
a sampling frequency setting portion for setting to the sampling signal generator circuit a sampling frequency such that a desired delay time can be obtained relevant to a phase of the signal under test,
wherein, in the sampling signal generator circuit, when the sampling signal having a cycle that corresponds to the sampling frequency is generated based on the common reference signal and the sampling frequency, the repetition cycle of the signal under test and the repetition cycle of the sampling signal are set based on a cycle of the common reference signal so that the repetition cycle of the sampling signal can be set independent of the repetition cycle of the signal under test.
In the thus configured waveform measuring apparatus, a reference signal oscillator for generating a reference signal that has a reference frequency employed for generating a repetition frequency of a signal under test is provided in a signal generator targeted for testing, the signal generator outputting a signal under test of which a signal waveform is measured by means of the waveform measuring apparatus.
In contrast, in the waveform measuring apparatus, a reference signal oscillator for generating a reference signal is not provided.
Then, a sampling signal is generated by employing a reference signal common to a reference signal outputted from a reference signal oscillator provided in a signal generator targeted for testing.
Therefore, the signal under test and sampling signal are generated by employing a common reference signal outputted from the same reference signal oscillator, thus making it possible to achieve the same advantageous effect as that according to the second aspect.
In addition, in order to achieve the foregoing object, according to an eighth aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the seventh aspect, further comprising a frequency measuring portion for measuring a repetition frequency of the signal under test, based on the reference signal output from the reference signal oscillator, wherein the measurement value of the repetition frequency of the signal under test measured by means of the frequency measuring portion is assigned to the sampling frequency setting portion.
In addition, in order to achieve the foregoing object, according to a ninth aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the seventh aspect, further comprising:
a power divider for, when the signal under test is an optical signal, power dividing the signal under test that is the optical signal in two ways;
a photo detector for converting into a signal under test of an electrical signal a signal under test that is one optical signal divided by means of the power divider; and
a frequency measuring portion for measuring a repetition frequency of a signal under test converted into an electrical signal by means of the photo detector, based on the reference signal output from the reference signal oscillator,
wherein the measurement value of the repetition frequency of the signal under test measured by means of the frequency measuring portion is assigned to the sampling frequency setting portion.
In addition, in order to achieve the foregoing object, according to a tenth aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the seventh aspect, further comprising:
a power divider for, when the signal under test is an optical signal, power dividing the signal under test that is the optical signal in two ways;
a clock recovery for detecting a clock of a repetition cycle from one signal under test divided by the power divider, thereby converting a signal under test that is the optical signal into a signal under test of an electrical signal having the repetition frequency and outputting the converted signal; and
a frequency measuring portion for measuring a repetition frequency of a signal under test converted into an electrical signal by means of the clock recovery, based on the reference signal output from the reference signal oscillator,
wherein the measurement value of the repetition frequency of the signal under test measured by means of the frequency measuring portion is assigned to the sampling frequency setting portion.
In addition, in order to achieve the foregoing object, according to an eleventh aspect of the present invention, there is provided a waveform measuring apparatus in accordance with the seventh aspect, further comprising:
a photo detector for, when the signal under test is an optical signal, and the sampling portion is an electro-absorption modulator, light receiving a signal under test of an optical signal sampled by means of a sampling signal of which a signal under test being the optical signal is inputted from the sampling signal generator circuit, and converting into a signal under test of an electrical signal a signal under test that is an optical signal after sampled;
an analog/digital converter for converting a signal under test converted into an electrical signal by means of the photo detector into a digital signal under test, thereby delivering the converted digital signal under test to the data processing portion; and
a display for converting a scale of a time axis of the enveloped waveform obtained by means of the data processing portion into a scale of an original signal under test, and displaying a signal waveform of the signal under test.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.